Mailserver blacklisted?

Hi
We have a mail server where we are hosting over 100 mail domains with over 1000 accounts
Runnig Imail 8.22 and ISP Spamfilter on our Windows 2003 Server.

One client in africa has reported several times that it seems like some of the once they sent mail to will not receive the mail as our mail server is blacklisted

This is the message they get in return. No other has this problem.
Can anyone se out from this if its reacting to our mail server and what trigger the rejection.
I have tried to check if our mail server is blacklisted on the net and I can not find any.
Hopefully someone can give me some directions on what the problem is: